Nature of band-A cathodoluminescence in CVD diamond films

Several sets of diamond films have been grown by Microwave Plasma-Enhanced Chemical Vapor Deposition (MWPECVD) using CH4-H2 and CO2-CH4 gas mixtures. The growth conditions were systematically changed in order to obtain different film morphologies, preferential orientations and crystal qualities. In particular, the CH4 concentration in the gas mixture was varied in a wide range to obtain different preferential orientations. Substrate growth temperatures as high as 950°C were used to intentionally obtain highly defective films. The resulting films were characterized by X-Ray Diffraction, Raman spectroscopy and Scanning Electron Microscopy. Cathodoluminescence has been investigated in the energy range 1.55-6.20 eV (200-800 nm). A strong dependence of the 2.85 eV (435 nm) band-A emission on the CH4 concentration is found. Moreover, this dependence does not substantially change with the growth conditions (substrate temperature, nucleation pretreatment, gas mixture etc.). The position of the diamond Raman peak is also correlated with the band-A emission. These results are discussed in terms of crystal defects and diamond film texturing induced by the growth process.